In conventional semiconductor fabrication processes, an optical lithography technology is usually employed to perform a lithography technique for forming required traces on a chip or a substrate. However, the optical lithography technology is subject to limitation on light diffraction and hardly applied to the case of having a line width smaller than 100 nanometers, thereby confining the development of line width. A next-generation lithography technology has been proposed, however it requires high device costs but has low yields. Therefore, a nanoimprint lithography (NIL) technology has been developed in recent years and widely used in lithographic processing, which can resolve the limitation on line width and has advantages such as high lithographic resolution, high fabrication rate and low production costs.
Generally, hot pressing molding process and ultraviolet curing molding process are the most prevalent nanoimprint techniques. The hot pressing molding process is to imprint patterns on a mold to a substrate coated with for example a polymer material under conditions of high temperature and high pressure. The ultraviolet curing molding process uses ultraviolet irradiation to cure and mold microstructures under conditions of normal temperature and normal pressure. These two processes both require uniform imprinting pressure to achieve satisfactory imprinting quality.
Referring to FIG. 6A, if the imprinting pressure is not uniformly applied, imprint depths of nanostructures 23 exerted by a mold 21 would be uneven, thereby causing partial distortion/deformation of the nanostructures 23. Referring to FIG. 6B, when the mold 21 is not perfectly parallel to a substrate 25, the nanostructures 23 in an imprint region are slanted, making the imprinting quality deteriorated. In the use of the hot pressing molding process, referring to FIG. 7A, a mold 21 fixed to an upper carrier 20 and having nanostructures 23 is employed during imprinting. The mold 21 is connected to a driving unit 50 driven by a power source and is moved toward a substrate 25 fixed to a lower carrier 30. However, this conventional pressing structure requires many mechanical components being stacked and assembled together. As the mechanical components are rigid, completely seamless contact between the mold, substrate, upper carrier and lower carrier cannot be achieved. Thus, the imprinting results as shown in FIG. 7B indicate quite uneven distribution of pressure. Such power transfer structure is difficult to satisfy the requirement of applying uniform pressure and causes unsatisfactory molding quality.
In view of the above problem of uneven pressure being applied, Puscasu et al. have proposed a reference entitled “Comparison of infrared frequency selective surfaces fabricated by direct-write electron-beam and bilayer nanoimprint lithographies” in November/December, 2000. Referring to FIG. 8, an upper plastic pad 27 is placed between an upper carrier 20 and a mold 21, and a lower plastic pad 29 is placed between a lower carrier 30 and a substrate 25. The plastic pads 27, 29 are softened when heating to a predetermined imprinting temperature, such that the plastic pads 27, 29 may provide relatively closer contact between the upper carrier 20 and the mold 21 and between the lower carrier 30 and the substrate 25 and uniform pressure can be applied during an imprinting process.
Since the above technology has nanoscale precision, quality of the imprinting process thereof must be more strictly controlled than that of the normal hot pressing molding process. However, the plastic pads under heating and pressure may encounter significant flowing deformation, thereby affecting transfer of imprinting force.
JP 2003-077867 discloses an imprint molding displacement mechanism. As shown in FIG. 9, the displacement mechanism uses a fulcrum 12 to give support to allow a substrate 25 to freely move on the fulcrum 12, and uses two spring elements 14 to carry two ends of the substrate 25. When a mold 21 is in contact with the substrate 25, the substrate 25 would automatically adjust its parallelism with respect to the mold 21 so as to achieve a function of applying uniform pressure. However, an imprinting device with this mechanism is only suitable for small area imprinting and has low yields; for large area imprinting, it would easily lead to substrate deformation especially at positions of the substrate relatively farther from the fulcrum. Further in the case of having excessive area for imprinting, the position of the substrate supported by the single fulcrum would be damaged due to stress concentration, making the function of applying uniform pressure hard to be conducted. Therefore, this conventional technology is still limited by size of imprinting area and magnitude of imprinting force though having the function of applying uniform pressure.
U.S. Patent Publication No. 20040219249 A1 discloses a nanoimprint uniform pressing apparatus. As shown in FIG. 10, the uniform pressing apparatus uses a uniform pressing unit 40 including a sealing membrane 40a made of a flexible material and a fluid 40b filled in a space formed by the sealing membrane 40a. The fluid 40b in the sealing membrane 40a has a property of each point with equal pressure so as to achieve effects of transferring uniform force and applying uniform pressure.
Although this apparatus does not encounter the problems in the foregoing conventional technology such as uneven pressure being applied, partial distortion/deformation of nanostructures, limited yields and stress concentration, the apparatus needs specific components for molding clamping, heating and cooling, and the uniform pressing unit 40 is not directly mounted on a mold or a substrate, such that a combination of components such as housing, heating/cooling unit and carrier unit must be used to transmit force to a molding region. As a result, transmission of uniform force would be affected by surface mechanical properties of the components (for example, surface roughness, parallelism, and mechanical processing accuracy).
In other words, the mechanical errors between different components must be precisely controlled to maintain satisfactory parallel arrangement between the mold and the substrate. This not only complicates the fabrication processes but also requires high assembly precision. Further as the uniform pressing unit 40 of the above apparatus is not directly mounted on the mold or substrate but is carried by a plurality of components, this causes difficulty in fabrication and assembly, and also makes the apparatus structure more complicated, thereby increasing the fabrication costs.
Therefore, in response to the various problems caused by uneven imprinting pressure and unsatisfactory parallelism during an imprinting process in the foregoing conventional technology and the difficulty in fabrication and assembly, it is greatly desirable to develop a uniform pressing apparatus, which can make each point in an imprint region have equal pressure, maintain satisfactory parallel arrangement between a mold and a substrate, and minimize influence of surface mechanical properties of other components on the parallel arrangement, so as to provide transmission of uniform imprinting pressure.